Rapid multiple property determination from bulk materials libraries prepared from chemically synthesized powders

A variety of high-performance materials are utilized in electrical, electronic, and mechanical systems. Such systems account for a significant fraction of the world’s electricity consumption. The next generation of such systems urgently require new material compositions which possess a better combination of both structural and functional properties. Only accelerated methodologies can rapidly determine the required multiple property set. Hence, a range of iron–cobalt–nickel ternary alloy composition powders were chemically synthesized. Compositionally graded bulk materials libraries were prepared by spark plasma sintering of these powders. A multiple property set of the crystal structure, magnetic, mechanical, and electrical properties were determined for a range of compositions. This property set revealed that a good combination of magnetic and mechanical properties can be obtained from Fe50Co40Ni10, high electrical resistivity from Fe54Co17Ni29 and high saturation magnetization as well as high hardness from Fe57Co29Ni14. Thus, this multiple property library, developed by accelerated methodologies, can be utilized to identify new ternary compositions satisfying diverse property sets relevant to next generation systems.

www.nature.com/scientificreports/ Fe-Co-Ni system were investigated. On the other hand, wet chemical synthesis [16][17][18][19] , especially the hydrazine reduction method, is becoming increasingly popular as a facile and low-cost process to produce powders 20 .
Hydrazine is a good metal reductant and produces nitrogen gas and water as by-products of the reaction, avoiding contamination issues which can degrade magnetic properties [21][22][23] . We performed chemical reduction synthesis, via the hydrazine reduction, to produce powders of a range of Fe-Co-Ni alloy compositions. This was followed by the preparation of compositionally graded bulk samples (materials libraries) by spark plasma sintering (SPS) of these powders. A significant advantage of this accelerated development method is that a single bulk material library (ML) consists of adjacent layers of several distinct compositions. Hence, a single ML can be used for multiple property measurements for a range of compositions. For example, the X-ray diffraction (XRD) patterns of the various compositions present in a compositionally graded bulk ML were obtained in a single run using a suitable XRD equipment.
The structural, magnetic, mechanical and electrical property measurements of a variety of compositions were determined to develop a multi-property data set of Fe-Co-Ni alloys. Specific compositions with attractive combinations of mechanical, magnetic and electrical properties for these ternary alloys were identified. In this study, ternary compositions were distributed across the ternary space of the Fe-Co-Ni phase diagram to cover different regions-face-centered cubic (FCC), body-centered cubic (BCC) and two phase (FCC + BCC)-and alloy compositions with promising properties were identified.

Experimental
Fe-Co-Ni powder synthesis. Iron  In a typical experiment for the synthesis of Fe-Co-Ni powder, the appropriate amounts of FeCl 2 ·4H 2 O, CoCl 2 ·6H 2 O and NiCl 2 ·6H 2 O for a given ternary alloy composition were weighed, placed in a flask and stirred vigorously until the metal chlorides dissolved in the solvent (consisting of EtOH and purified water in the ratio 3:1). A 4 M NaOH solution was then added, followed by hydrazine monohydrate. The molar ratio of metal chlorides to NaOH to hydrazine monohydrate was approximately 1:2.5:16. The flask was then sealed, with a needle inserted to allow the evolved gases to vent, and the temperature was maintained at ~ 60 °C for 1 h. The black particles which formed were washed a few times with ethanol to remove the by-products. A permanent magnet was used to collect the black particles, which were then placed in a vacuum oven to form dry powders. The conversion yield to powders from each synthesis was more than 90%.
Compositionally graded Fe-Co-Ni alloy using spark plasma sintering (SPS). Compositionally graded bulk samples were prepared by SPS by consolidating the powders of a given composition as an individual layer, as described previously 15 . A tantalum foil was used as an inert spacer between each layer. Spark plasma sintering was performed in a Fuji Electronic Industrial SPS-211LX equipment at a vacuum level below 8 Pa under a pressure of 40 MPa at 950 °C for 15 min. Two vertically cut sections from the sample were prepared. One of the sections was labelled "as-SPS", while the other section was annealed at 1000 °C for 2 h in 95% Ar + 5% H 2 atmosphere and labelled "annealed". Characterization techniques. The morphology of the chemically synthesized alloy powders was studied using a JEOL JSM-7600F field emission scanning electron microscope (FESEM). Elemental mapping was performed by an energy dispersive X-ray (EDX) spectrometer attached to the FESEM. The crystal structures of the as-SPS and annealed samples were determined by the XRD technique using a Bruker D8 Discover diffractometer (CuK α radiation, λ = 0.154 nm). Phase fractions calculations were done via Rietveld refinement in TOPAS V6 24 software. The Curie temperature (T c ) was measured by a previously described method of conducting a thermogravimetric analysis (TGA) run of the sample while placing a permanent magnet near the TGA pan 15,25 , using a TA Instruments Q600 SDT. The magnetic properties were measured using a physical property measurement system (PPMS, EverCool-II, Quantum Design) equipped with a vibrating sample magnetometer (VSM) attachment.
The microhardness of the compositionally graded samples was measured using a Vickers hardness tester (Future-Tech) at a load of 1 kgf. A four-point probe tester (Keithlink) was used to obtain the resistivity (ρ), by applying the following equation 26,27 : where V, I, t and s are the voltage, current, sample thickness and probe spacing respectively.
The graphical data presented in this work were plotted in Origin(Pro) 2020b 28 .

Results and discussion
Size and morphology of powders. Figure 1a shows the various compositions, in atomic percentage (at%), of Fe-Co-Ni powders synthesized in this work. Ten compositions from different regions of the ternary phase diagram were synthesized, and high throughput approaches were used to investigate the magnetic, mechanical and electrical properties. Sufficient mass of powders was produced from this synthesis method to form com- www.nature.com/scientificreports/ positionally graded bulk samples. Figure 1b shows the nominal compositions as well as the composition values obtained using EDX. For ease of discussion, the samples will be referred to by their nominal compositions. The motivation to choose these specific compositions was to explore the various phase fields-face-centered cubic (FCC), body-centered cubic (BCC) and two phase (FCC + BCC) of the Fe-Co-Ni system. One of these compositions, Fe 54 Co 17 Ni 29 is known as "Kovar" and is technologically important in the electronics industry 16,17 .
The scanning electron micrographs of the chemically synthesized ternary alloys are presented in Fig. 2a-j. The alloys generally formed spheres or spherulites and were in the particle size range of 55-750 nm. The average size of the particles with respect to composition is shown in Fig. 2k. Cobalt-rich compositions had larger particles sizes with more flower-like features-this morphological variation could be attributed to differences in the reduction rates of the chemical reduction reaction 18 . Figure 3a,b

Structure and Phase Analysis of compositionally graded bulk materials libraries.
shows the X-ray diffraction patterns of the as-SPS and annealed Fe-Co-Ni materials libraries. Depending on the composition, the crystal structures were either FCC, BCC or a mixture of both phases. In Fig. 3a, the as-SPS samples displayed varying degrees of crystallinity, with some compositions displaying diffraction peaks with much higher intensities. The diffraction peaks matched the 111, 200 and 220 peaks of the FCC phase, and the 110 and 200 peaks of the BCC phase. For the cases of Fe 54 Co 17 Ni 29 and Fe 57 Co 29 Ni 14 , the intensities of the diffraction peaks were low, and minor phases of Fe 2 O 3 and Fe 0.5 Ni 0.5 were detected (see Fig. 3a inset).
After annealing (Fig. 3b), all samples exhibited sharp diffraction peaks, and the minor phases were absent. Annealing in gas containing hydrogen is beneficial for removing impurities such as oxygen 29 , and in dissolving the minor Fe 0.5 Ni 0.5 phase back into the matrix. The crystal structures generally remained the same, but two compositions showed some changes: for Fe 54 Co 17 Ni 29 , an additional BCC phase formed, in addition to the initially present FCC phase, while for Fe 57 Co 29 Ni 14 , the FCC phase disappeared, leaving only the BCC phase. The weight % (wt%) of these phases in as-SPS and annealed samples are presented in Fig. 3c,d, respectively. As-SPS Fe 54 Co 17 Ni 29 exhibited a FCC structure and contains ~ 14 wt% of Fe 2 O 3 and 19 wt% of Fe 0.5 Ni 0.5 . As-SPS Fe 57 Co 29 Ni 14 is twophase, with FCC (16.6 wt%) and BCC (65.9 wt%) and contained 17.5 wt% Fe 0.5 Ni 0.5 . After annealing, 33.3 wt% of the BCC phase was present in Fe 54 Co 17 Ni 29, and the minor phases in both samples disappeared. This showed that heat treatment could be used to control the phases present in these ternary alloys. These compositions and phases match earlier reports for the FCC, BCC + FCC and BCC phase field regions 2,30 . Figure 4a,b shows the field dependence of magnetization at room temperature for the as-SPS and annealed samples. The variation of saturation magnetization (M s ) with composition is shown in Fig. 4c. After annealing, the M s increased; there was a 53% and 41% increase for the Fe 54 Co 17 Ni 29 and Fe 57 Co 29 Ni 14 compositions, respectively. This was due to the removal of the minority phases (of Fe 2 O 3 and Fe 0.5 Ni 0.5 ) in Fe 54 Co 17 Ni 29 and Fe 57 Co 29 Ni 14 , as suggested by the XRD results.

Magnetic properties.
For the alloy compositions with the same Fe content, those with higher Co content compared to Ni content exhibited higher M s due to the larger atomic magnetic moment of Co compared to Ni (1.7 μB vs 0.6 μB) 31 . For the annealed samples, a high M s of 207 emu/g and 210 emu/g was obtained for Fe 50 Co 40 Ni 10 and Fe 57 Co 29 Ni 14 , respectively. The M s values obtained in this work for equi-atomic Fe-Co-Ni were close to the reported value 18 but higher than the reported values for Fe 40 Co 20 Ni 40 16 and Fe 50 Co 40 Ni 10 17 , although it was noted that those samples were in powder form.
The coercivity (H c ) was also obtained from the M-H curves, as shown in Fig. 4d. These values decreased or remained the same for most samples after annealing, except for Fe 10   www.nature.com/scientificreports/ Figure 5 shows the values of T c versus composition in the ternary alloy samples. The T c varied over a wide range, from 477 to 962 °C, making this study relevant to a variety of applications. Compositions that were rich in Fe and Co, with Ni between 10 and 20 at%, had higher T c values. The T c values for most samples matched closely with those reported in literature, while unique values were unavailable for Fe 40 Co 50 Ni 10, Fe 50 Co 40 Ni 10 and Fe 57 Co 29 Ni 14 32,33 . For these three compositions, more than one magnetic transition may occur, depending on the processing temperature, hence more than one value of T c has been reported. Figure 6a,b shows the Vickers hardness (H v ) and electrical resistivity (ρ) of the as-SPS and annealed samples. For the as-SPS samples, the microhardness varied greatly with composition, with the highest value of 439.5 H v for Fe 50 Co 40 Ni 10 , followed closely by 415.9 H v for Fe 40 Co 50 Ni 10 . After annealing, however, the microhardness decreased in all but two samples, Fe 20 Co 40 Ni 40 and Fe 57 Co 29 Ni 14 , with these two compositions exhibiting a slight increase of 7.5% and 6.7%, respectively. For the case of Fe 57 Co 29 Ni 14 , this could be due to the disappearance of the FCC phase, which is known to be softer than the BCC phase 34 . A decrease in microhardness could also be due to lower residual stresses and defects 35 . This effect was most pronounced for  Electrical properties. The electrical resistivity (ρ) of magnetic materials is important for controlling the power loss in rotating electrical machines. The eddy current loss is inversely proportional to the resistivity of the material. Figure 6b shows the values of electrical resistivity for the various compositions. Unexpectedly, the as-SPS samples generally exhibited values of electrical resistivity equal to or lower than those of the annealed samples, except for Fe 33 33 showed that the trend of electrical resistivity values closely matched the literature, but the absolute values were about 2-5 times higher, likely due to the different processing method used here for sample preparation as compared to the samples reported in the literature. It is also known that electrical resistivity can change as a function of annealing temperatures due to relaxation processes and phase transformations 36 ; the high annealing temperature of 1000 °C may have led to higher resistivity values. Annealed Fe 57 Co 29 Ni 14 exhibited the opposite trend for the change in microhardness and electrical resistivity compared with the other samples. This decrease in electrical resistivity could be attributed to a phase change, as shown by the XRD results. Scanning electron microscopy was performed on the three annealed samples which exhibited larger change in electrical resistivity-both secondary electron images and backscattered electron images were taken. For Fe 40 Co 20 Ni 40 in Fig. 7a,b, pores of various sizes were observed. The increase in electrical resistivity was likely to be due to these pores. For Fe 54 Co 17 Ni 29 , the electrical resistivities of the minority phases was much higher for Fe 2 O 3 37 and slightly lower for Fe 0.5 Ni 0.5 compared to the electrical resistivity of the ternary alloy 33 . Despite the removal of these minority phases, there was an increase in electrical resistivity due to the presence of multiple pores of various sizes and small and shallow cracks (Fig. 7c,d). For Fe 57 Co 29 Ni 14 , there was less porosity, but some surface roughness, as observed in Fig. 7e. Backscattered electron imaging in Fig. 7f showed that these pores were shallow and most of the alloy was still a continuous phase. Coupled with the removal of the Fe 0.5 Ni 0.5 minority phase (which had higher electrical resistivity than this alloy composition 33 ), the effect was a lower electrical resistivity value after annealing. The density of pores for Fe 40 Co 20 Ni 40 , Fe 54 Co 17 Ni 29 and Fe 57 Co 29 Ni 14 were measured using ImageJ 38 and found to be ~ 1.1%, 4.2% and 4.6%, respectively. This porosity could be due to processing conditions and also the removal of impurity phases during annealing. Thus, changes in the phase fraction and defects, e.g. pores or cracks, can significantly affect measured electrical resistivity values of the    www.nature.com/scientificreports/ samples. To reduce the porosity, higher sintering temperature and pressure can be used in SPS. The particle size of starting materials can also be tuned to minimize porosity in the sintered sample.

Mechanical properties.
Multiple property set obtained from the materials library. Figure 8a- This series of colour maps shows that accelerated methodologies can provide the multi-property set required for the identification of novel compositions possessing the required combination of properties. Broadly, the three compositions mentioned above exhibited promising combinations of properties. Figure 8f shows the radar plot comparing normalized M s , H c , H v , ρ, T c and cost values for the annealed samples. Of the three above-mentioned compositions, Fe 54 Co 17 Ni 29 was the most cost effective. Selection can be readily performed for specific property requirements of a given application. For example, materials with higher resistivity could be considered for high frequency applications, while materials with higher T c could be deployed in high temperature applications. www.nature.com/scientificreports/ A high M s of 207 emu/g was obtained for Fe 50 Co 40 Ni 10 , which is more than that of permalloy, the T c was also 58% higher than permalloy 15 . The identification of such specific compositions in a vast ternary composition space underlines the advantage of our accelerated methodology. Further, chemical synthesis could be used to produce a wide variety of ternary alloy compositions.
In this accelerated methodology, we could quickly obtain valuable trends in the properties although a quantitative match with the conventional method was not always obtained. The absolute values of the mechanical and electrical properties of the compositions of interest were higher than previous literature on related permalloy-Co alloys 15 due to differences in the compositions and the synthesis method. Electrical resistivity was a very sensitive function of several defects at various length scales and annealing had different effects on each composition. Hence there was a quantitative difference in the properties assessed by the accelerated and conventional methods.

Validation.
Validation was performed on the three selected compositions from Fe 50 Co 40 Ni 10 to Fe 57 Co 29 Ni 14 that were predicted to exhibit a promising combination of properties. After synthesis and drying, the ternary alloy powders were compacted individually in SPS. The obtained pellets were then cut into half for annealing at 1000 °C. Earlier samples prepared by accelerated methodologies were labelled "High TP" for high throughput, validation samples were labelled "Validation". As seen in Fig. 9a In terms of the electrical resistivity (Fig. 9c), the same trend was maintained in the validation samples compared to the samples prepared via the high throughput method. However, the absolute values were lower here for Fe 50 Co 40 Ni 10 and Fe 54 Co 17 Ni 29 (at 37.5% and 62.7% respectively), but 14.9% higher for Fe 57 Co 29 Ni 14 . The unusually high electrical resistivity value obtained earlier for Fe 54 Co 17 Ni 29 may have been due to pores or defects present in compositionally graded samples. This reinforced the earlier observation that electrical resistivity is www.nature.com/scientificreports/ sensitive to defects and pores of various length scales. These electrical resistivity values were higher than the values reported by Bozorth 33 . Vickers hardness trends in Fig. 9d were similar to those obtained by the high throughput method. The value was 3.2% less than the high throughput value for Fe 50 Co 40 Ni 10 , 42% more than the high throughput value for Fe 54 Co 17 Ni 29 and 11% more than the high throughput value for Fe 57 Co 29 Ni 14 . The high hardness of Fe 54 Co 17 Ni 29 validation samples was likely to be due to less defects in the validation sample. Overall, the trends in the validation studies were like those seen in the accelerated methodology. Compared to the material properties of the commercially available materials, except for H c , which is higher, most of the other values obtained here are comparable.

Conclusions
Hydrazine reduction synthesis was used to synthesize sufficient mass of powders for rapid multi-property evaluation of a range of ternary Fe-Co-Ni alloys. The powders were used to prepare compositionally graded bulk samples via SPS. These samples were evaluated by rapid structural characterization and multi-property assessment. A material property data set was developed, and a good balance of properties was identified in the composition region between Fe 50 Co 40 Ni 10 and Fe 57 Co 29 Ni 14 . The crystal structures were either BCC or BCC + FCC. Compaction of ternary alloys into a bulk material and subsequent annealing in reducing gas improved magnetic properties. The Fe 50 Co 40 Ni 10 , Fe 54 Co 17 Ni 29 and Fe 57 Co 29 Ni 14 compositions were identified as possessing an interesting property mix. Results from the validation experiments were qualitatively like the high throughput results. Thus, an accelerated methodology to construct a multiple property data set from chemically synthesized powders processed into bulk samples (materials libraries) was successfully carried out and promising new alloy compositions identified.

Data availability
The data produced and analyzed during the current study are available from the corresponding author on reasonable request.